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Scientists have found that the genetic quality of sperm decreases with age.

A study at the Lawrence Livermore National Laboratory at Berkeley found that DNA can fragment in older men and the frequency of a mutation which causes dwarfism rises.

The work, published this week in Proceedings of the National Academy of Sciences, used samples from 97 volunteers aged 22 to 80. The team "filtered" the sperm using a technique called flow cytometry, which can detect damaged DNA. Dwarfism was identified by an amplification technique sensitive to mutated copies of the gene.

They found that the risk of a man passing on a mutation causing dwarfism rises by about two per cent each year, and broader likelihood of older men fathering a successful pregnancy falls away with the DNA fragmentation.

Co-lead author Barbara Eskenazi said: "We know that women have a biological time clock. Our research suggests that men too have a biological time clock - only it is different. Men seem to have a gradual rather than an abrupt change in fertility and in the potential to produce viable healthy offspring."

Scientists have long known that older eggs are more likely to suffer large scale abnormalities across large sections of DNA or whole chromosomes, as in Down Syndrome. The researchers did not find that this kind of genetic problem increases with the father's age.

Co-lead author of the study Andrew Wryobek warned men to be aware their age could affect their children. He said: "This study shows that men who wait until they're older to have children are not only risking difficulties conceiving, they could also be increasing the risk of having children with genetic problems." ®